Microbial histidine decarboxylases. We continue to study the process by which a single chain precursor subunit of this enzyme is activated to form the dimeric subunit (one chain of which contains the newly formed pyruvoyl moiety) characteristic of the native enzyme. To what extent is the tertiary structure of the proenzyme essential for the activation process? How can the unusual dependence (half-order) on monovalent cations for this activation reaction be explained? What is the nature of the difference between the wild type Lactobacillus 30a that does not accumulate proenzyme, and the mutant of this enzyme that does? Are there differences in the amino acid sequences of the activated proenzyme and the wild type enzyme? These and similar questions will occupy most of our time on this project. Histidinol-P Aminotransferase. We have now worked out a procedure for following this reaction in the physiological direction. We are now in position to determine the kinetics of this reaction, which we stated as one of our objectives last year.